According to a communication industry development, speech communication has gradually developed from a wire terminal communication system connected to a wire communication channel to a radio terminal communication system using a radio telephone.
Generally the radio telephone is classified as either a portable radio telephone utilizing a radio channel or a cordless phone replacing a cord between a base unit and a hand-set of a telephone with a radio channel.
The radio telephone and cordless phone use a rechargeable battery for operating an internal electronic and electrical circuit because a user carries a portable device for speech communication. Accordingly the battery of the portable device should be charged if a voltage of the battery is dropped below a given level of voltage. The battery of the portable device of the radio telephone is generally charged by a charger. A charging function of the radio telephone is performed by mechanically contacting a voltage output end of the charger with a charging end of the portable device for conducting electrical energy. For example, a battery of the portable device is charged by contacting a conductive voltage pad of the battery with an output end of a terminal spring of a charger.
The conductive voltage pad of the battery and the output end of the charger, however, are made of a conductive metal, so that such mechanical contact is not proper for charging the battery efficiently because the conductive metal is apt to be oxidized easily and an alien substance is apt to adhere to the voltage pad of the battery and the output end of the charger.
The cordless phone having a base unit and a portable device transmits and receives data representing an identification code (ID code) therebetween and a state of communication at a given period or at a time of occurrence of a corresponding event is performed. Data communication is performed between the base unit and the portable device by a radio frequency transmitter and receiver while the portable device is separated from the base unit. When the portable device is placed on the base unit, data communication is performed between the two through another communication terminal, after cutting off power to the radio frequency transmitter and receiver in order to save the battery's energy.
Both the base unit and the portable device have a data communication terminal made of a good conductive metal pad, and data communication is performed during a state of mechanical contact when the base unit and portable device are coupled to each other.
However, in the case of either communication terminal being oxidized and/or contaminated with an alien substance, it is impossible for a conventional radio telephone to perform data communication because of bad electrical contact. Further, because the conventional radio telephone has only communication terminal between the base unit and the portable device, data transmission speed is slow due to the fact that full duplex communication is impossible.